Reader Henry Spender saw a foamy substance collecting against a fallen tree while walking next to the Avon near Five Rivers Leisure Centre on Boxing Day.
He sent a picture to the Journal and said: “While walking along the recently reopened boardwalk, I saw this foam swirling around and wondered what it is.
“There was more upriver. Can anyone clarify the cause?”
The Environment Agency has explained why foam can appear on rivers and still water – and says it is, usually, a natural phenomenon.
A spokesperson said natural foam mostly consists of air and water.
The light brown colour suggests this foam occurred naturally (Image: Henry Spender) “Foam is produced when molecules act as ‘surfactants’, interfering with the surface tension of water and allowing air and water to mix more easily,” they said.
“Naturally produced surfactants are released from decaying organic material, but are also released in small amounts by living organisms.
“These molecules often float on the surface of the water as a thin film.
“Turbulence from waves, currents and wind cause the surfactant molecules to trap small bubbles that make up the foam.
“Large amounts of foam can accumulate in certain areas of the river, especially downwind and within eddies, or just downstream of turbulent water, for instance below weirs.
“This foam is usually harmless.”
However, some foam can be caused by pollutants such as detergents.
Natural foam normally turns light brown as it collects sediment and other organic matter and can have a “natural, earthy, fishy or fresh cut grass smell”.
It can persist for “some time” but will gradually shrink. It is usually found on windy days or after rain storms.
Man-made foam, on the other hand, will normally be white and smell “fragrant, perfumed or soapy” and usually dissipates quickly.
Anyone who believes they have found pollution should contact the Environment Agency.